1. Field of the Invention
The present invention relates generally to control elements used in electronic circuits and, more specifically, the present invention relates to control elements with integrated power transistors.
2. Background Information
Two of the primary goals in the design of control elements with integrated power transistors are cost and performance. Cost is generally reduced when the number of external components required in the electronic circuit are reduced, and when smaller, more efficient power transistors are employed. Performance may be improved by adopting a more efficient power transistor, which increases efficiency, and by lowering the manufacturing variance, which allows better control of critical parameters such as the peak current delivered by the power transistor.
FIG. 1 shows a power supply 10, which is provided as an example of an electronic circuit using a control element with power transistor. The control element 22 for this known power supply 10 includes a control circuit 23 and a separate power transistor 21. In power supply 10, the start-up function is performed by resistor 32, which provides the high voltage DC from bridge rectifier 12 to the control circuit 23. Unfortunately, resistor 32 is expensive, requires a large area in the power supply and lowers supply efficiency by dissipating power continuously, even after the start-up function is completed. The current limit function of power supply 10 is provided by a sense resistor 33 that is in series with the source of power transistor 21. The voltage across resistor 33, which increases with increasing current through power transistor 21, is coupled to the control circuit 23. When the current through power transistor 21 reaches a predetermined level, the control circuit 23 turns of power transistor 21. Drawbacks of this approach are the cost, size and power dissipation of resistor 33.
FIG. 2 shows a known power supply 50 similar to power supply 10, except that resistor 32 has been eliminated. A voltage regulator internal to power supply chip 52 now performs the start-up function. The voltage regulator in power supply chip 52 may be turned off after the start-up function is completed, thus eliminating the extra power dissipation inherent to power supply 10. However, the voltage regulator in power supply chip 52 includes a high-voltage offline transistor 54 that consumes a significant area on power supply chip 52 and is also prone to electrical static discharge (ESD) and safe operating area (SOA) damage.
FIG. 3 shows a known power supply 70 that avoids some of the problems of power supplies 10 and 50. Power supply 70 does not require a start-up resistor 32 or a high-voltage offline transistor 54. Instead, a tap 90 at the junction between junction field effect transistor (JFET) 86 and insulated gate field effect transistor (IGFET) 88 of metal oxide semiconductor field effect transistor (MOSFET) 84 is used to perform the start-up function. Tap 90 may also be used to monitor the voltage for performing the current limit function, obviating the need for a separate sense resistor. The main limitation of this approach is that the MOSFET 84 used in power supply 70 is limited to one that can be monolithically integrated on the chip 82. In some cases, discrete power devices with higher efficiency may be available, but cannot be employed in the monolithic approach of power supply 70.